Resin cement is increasingly being used as a dental luting material for cementing prosthetic devices to a tooth, due to its excellent mechanical properties and good optical properties. Resin cement has good translucency and good initial color-matching to a tooth's natural color. Cementing prosthetic devices such as veneers, inlays, onlays, crowns, bridges, and posts to a tooth using a resin cement involves the steps of (1) preparing the tooth (cutting and cleaning) to receive the prosthetic device; (2) optionally, etching the prepared tooth surface with an acidic etchant and rinsing to remove the etchant; (3) applying an acidic primer/adhesive to the tooth surface receiving the prosthetic device; (4) adhering the prosthetic device to the primer/adhesive coated tooth surface using a resin cement, and (5) hardening the dental cement. Adhesion of the prosthetic device depends on the extent of curing of the primer/adhesive and cement. When cementing metal-based or high opacity prosthetic devices, it is difficult for light to reach the adhesive and cement, so a photocure process cannot be used. Instead, a redox initiator system must be used for the cement to cure effectively in the dark without the need for light.
Current resin cements use benzoyl peroxide (BPO) and a tertiary amine as the redox initiator system to initiate polymerization and cure the cement. The resin cement typically comprises a base paste containing the tertiary amine and a catalyst paste containing BPO. When the base and catalyst pastes are mixed, BPO and the tertiary amine form a redox pair generating free radicals and initiating polymerization of the methacrylate monomers, causing the cement to harden (cure).
Most primer/adhesives, particularly self-etching primer/adhesive systems, are acidic. When a resin cement containing a BPO/tertiary amine initiator system is applied to a tooth coated with such an acidic primer/adhesive, the tertiary amine is quickly neutralized by the acidic primer/adhesive, thereby compromising its curing efficacy and resulting in poor adhesion at the primer/cement interface. The incompatibility of current resin cements with acidic primers/adhesives has been reported (King et al, Incompatibility Profiles of All-In-One Adhesives. I. True vs Apparent Incompatibility., International Association for Dental Research 82nd General Session, Abstract No. 23, Mar. 10-13, 2004, Honolulu, Hi.). This same incompatibility also exists between a core buildup material and an acidic primer/adhesive.
A redox initiator system that can tolerate the acidity of the primer/adhesive and that cures effectively at the interface between a cement and the acidic primer/adhesive is desirable. One such redox initiator system utilizes 1-(2-pyridyl)-2-thiourea and a tertiary hydroperoxide in a self-adhering dental composition (Qian U.S. Published patent application Ser. No. 10/440,804). This composition was applied directly to the tooth without any pre-treatment (etching or priming) of the tooth surface, but the bond strength was low without a primer/adhesive.
Other compositions are thus desirable.